In this research proposal we will attempt to establish if altered rates of protein renewal are involved in the deterioration of cardiac muscle function that occurs after severe or prolonged work overload. In addition we hope to obtain information on how multiprotein structures such as myofibrils are assembled and degraded. The major objectives are: 1) To develop procedures to measure the true synthetic rates and half-lives of cardiac muscle proteins. The approach will use rigorous kinetic and statistical analysis of precursor-product relationships. We will modify the mathematical treatment of data in current use so that calculations are based on the measurement of the direct protein precursor, aminoacyl tRNA, rather than intracellular free amino acid. We will also try to measure the fractional synthetic rates of individual muscle proteins independent of reincorporation of label released by protein degradation. 2) To examine mechanisms of cardiac myofibrillar assembly in vivo. We will evaluate the possibility that pools of myofibrillar proteins exist that are not yet assembled into myofibrils. 3) To study the process by which myofibrillar proteins are degraded. We wish to evaluate the participation of lysosomal acid hydrolases and neutral proteases in the process of intracellular protein degradation. Neutral proteases and the Ca ion -activated protease which removes Z-bands will be characterized. Studies will be carried out on hearts of intact animals subjected to conditions that lead to rapid changes in myocardial mass (hypertrophy or regression of hypertrophy), or that result in myocardial damage: ischemia, hypoxia. For this work it is necessary to distinguish between changes in muscle and connective tissue cells. We will therefore try to standardize the procedures for preparation of single myocytes from adult myocardium. BIBLIOGRAPHIC REFERENCES: Fischman, D.A., Doyle, C.M., and Zak, R. In: Perspectives in Cardiovascular Research I. Editors, M. Lieberman and Toyomi Sano, New York, Raven Press, 1976, p.67. DNA synthesis in Chick Cardiac Muscle: Comparative Observations of In vivo and In vitro Growth. Etlinger, J., Zak, R., and Fischman, D.A. J. Cell Biol. 68, 123 (1976). The Composition and Disassembly of Myofibrils of Vertebrate Striated Muscle.